This invention relates to neurosurgery in general, and in particular to a method and system for the accurate anatomical localization of functional activity in the human brain during a neurosurgical procedure. It has been long known that the sensory, motor, and cognitive functions of the brain are organized in anatomically distinct locations. However, with respect to any particular brain function there is considerable variation in anatomical localization between individuals. During a neurosurgical procedure it would be of obvious value to be able to reliably determine whether complete or partial removal of a particular region of the brain would cause a subsequent functional deficit in the patient. For example, before surgical resection, or removal, of a region of abnormal tissue in a patient's temporal lobe causing repeated epileptic seizures, it would be desirable to localize the language region of the particular patient's brain in order not to damage or destroy the subsequent ability of the patient to speak because of inadvertent resection of a functionally active region of tissue in the temporal lobe. Other types of neurosurgical procedures, such as tumor resection, would also benefit from a precise method for functional localization before the neurosurgical procedure is actually performed.
Current methods for localization of brain function may be divided into two general types: invasive and noninvasive. While noninvasive methods of imaging such as positron emission tomography, regional cerebral blood flow, magnetoencephalography, and brain electrical activity mapping have been used for general anatomical localization of brain function, each of these methods has a significant flaw precluding accurate and effective use for localization in the neurosurgical setting. Positron emission tomography, which uses radioactive positron-emitting isotopes such as .sup.18 F-fluorodeoxyglucose to create functional maps of brain metabolism and blood flow, does not possess the spatial resolution necessary for precise neurosurgical localization. Likewise, the measurement of regional cerebral blood flow using .sup.133 xenon is implemented using large crystal detectors and thus provides very low spatial resolution. The anatomical relationship of non-invasively generated brain electrical and magnetic activity maps to the underlying electrophysiological generators is variable and may be confounded by differences in the spatial orientation of the underlying generators.
There are, however, two invasive methods which are currently used for functional localization in the brain during neurosurgery. These are a) electrocorticographic recording of the electroencephalogram (EEG) and a sensory evoked potentials, and b) electrical stimulation of the brain. During electrocorticography the spontaneous ongoing electrical activity of the brain is monitored using an array of electrodes placed on the surface of the brain and the extremely low-amplitude analog signals, on the order of microvolts, are recorded with galvanometric pens on continuous chart paper. The traces are then analyzed visually and the location of maximum activity or of particular EEG patterns is estimated. Evoked potentials are recorded by monitoring the EEG with an array of electrodes, time-locking digitization of the analog EEG signals to the presentation of a repetitive sensory stimulus, and averaging multiple stimulus trials to reduce background noise. Components of the EEG signal which are temporally synchronized with the stimulus are augmented by the signal averaging process, while components of the EEG which are not temporally synchronized are largely eliminated by the averaging process. Multiple channels of averaged signals are then presented on a video monitor or on chart paper, the traces are analyzed visually, and the location of maximum electrical activity evoked by the sensory stimulus is estimated.
The disadvantage of electrocorticography is that while some spontaneous EEG patterns are well correlated with anatomical landmarks, these patterns may not be present in all patients. Also, for most brain functions of interest no specific EEG pattern has been identified. The disadvantage of recording evoked potentials for neurosurgical localization is that these potentials are mainly useful in defining the somatosensory region, and are of limited use in the localization of other higher brain functions. In addition, both electrocorticography and evoked potential measurements are susceptible to volume conduction effects, in which activity may be recorded at sites relatively distant from the actual electrophysiological generators.
Localization of brain function using electrical stimulation is implemented through the direct application of short current pulses of alternating polarity to different regions of the brain and observing effects of the stimulation on the patient such as muscle twitching during stimulation of the motor region, or obliteration of the ability to speak during stimulation of the language region. Electrical stimulation is useful in the localization of the somatosensory, language, and other regions of higher brain function, but is very time-consuming, and the resolving power of the technique is limited by the size and number of the stimulating electrodes. In addition, the technique of electrical stimulation has been criticized as physiologically artificial, in the sense that being subjected to externally applied electrical current is obviously not a normal physiological condition of the brain. Also, the possibility of current leakage to tissue other than that which is being stimulated cannot be excluded. Finally, the possibility exists that repetitive electrical stimulation of the brain may induce harmful seizures or cause permanent damage to brain tissue.
It is thus obvious that the present state of the neurosurgical art does not offer a dependable, accurate system for localization of brain function which can be used during a neurosurgical procedure. It follows that a system and method for functional localization which combined high spatial resolution, anatomical accuracy, and real time data acquisition and analysis would satisfy an urgent need in current neurosurgical practice. It is the object of the present invention to provide such a system and method.